High Resolution X-ray Scattering Studies of Structural Phase Transitions in BaFe$_{2-x}$Cr$_x$As$_2$

While the effects of electron-doping on the parent compounds of the 122 family of Fe-based superconductors have been extremely well-studied in recent years, far less is known about the influence of hole-doping in compounds such as BaFe$_{2-x}$Cr$_x$As$_2$. In contrast to the electron-doped 122 systems, the hole-doped compounds do not become superconducting. Furthermore, while the hole-doped compounds exhibit similar structural and magnetic phase transitions, they appear to be much less sensitive to dopant concentration. We have performed high resolution x-ray scattering and magnetic susceptibility measurements on single crystal samples of BaFe$_{2-x}$Cr$_x$As$_2$ for Cr concentrations ranging from 0 $\le$ x $\le$ 0.67. These measurements allow us to determine the magnetic and structural phase transitions for this series and map out the low temperature phase diagram as a function of doping. In particular, we have carried out detailed measurements of the tetragonal (I4/mmm) to orthorhombic (Fmmm) structural phase transition which reveal how the orthorhombicity of the system evolves with increasing Cr concentration and how this correlates with the values of T$_s$ and T$_m$.